I have purchased lengths of tubing( from Rio) and cut them to
length. To make a seat I put a smaller tube inside the larger and
cut it slightly shorter. I have also vulcanized the results after
attached to a design. Darryl

I suppose that If I purchase the tubing, I should get one
diameter larger than the stone I need? (for the outside) I don't
cast at all, so vulcanizing wouldn't be something that I even
know how to start to do.

Do you have trouble getting a snug fit of one tube within the
other? Do you still have to burr the setting to fit the stone
perfectly, or do you have such a variety of tubing that you can
always find the size(s) you need? Do you sometimes have to draw
tubing to the correct size?